For the generation of direct voltages from AC voltages, at the present time mostly rectifier bridges are used which are made up of an interconnection of diodes.
The conversion of an AC voltage to a DC voltage takes place in a motor vehicle, for instance, in which an AC voltage is generated by a generator which is converted by a post-connected rectifier bridge to a DC voltage.
An example for such a voltage supply of a motor vehicle is shown in FIG. 1. This includes a generator device GEN which has an excitation winding G and radially connected phase windings U, V and W. The phase voltages provided at phase windings U, V and W are supplied to a rectifier bridge RF, which provides the desired DC voltage at its output. Rectifier bridge RF includes three bridge branches. In each of these bridge branches two silicon p-n diodes D are provided.
Conditioned physically, during a forward operation of a silicon p-n diode in rectifier applications, diode forward voltages of ca. 800 mV to ca. 2 V come about. These diode forward voltages, at meaningful dimensioning, are usually not able to be lowered below ca. 1 V. Therefore, especially in the case of the rectification of lower alternating voltages, power losses are created at the rectifier diodes. In generators used for the voltage supply of passenger vehicles, the alternating voltages that are to be rectified usually amount to about 17 V peak-to-peak between the terminals of two phase windings. Since the current flows via two rectifier diodes, because of the rectification, a voltage reduction is created averaging about 2 V. In this example, the power loss in the rectifier at corresponding load amounts to about 20% of the power output. The power loss converted in the rectifier has to be eliminated in the form of heat by costly cooling elements. In addition, the power losses directly affect the fuel consumption of the respective vehicle.